The present invention relates generally to thin film optical filters, and more specifically to a novel rugate type gradient-index thin film filter.
Rugate filters are thin film optical interference filters in which the index of refraction of the film varies sinusoidally with film thickness. Rugate filters differ from conventional thin film filters, such as quarter-wave stacks, in that the index of refraction varies continuously, i.e., gradient-index, with thickness, instead of discontinuously as in a quarter-wave stack of alternating discrete layers of high and low refractive index materials.
Thin film optical interference filters, depending upon their construction, may operate either as antireflection coatings or as highly reflective coatings. Appropriately constructed reflective coatings are commonly used as narrow bandwidth blocking filters that reflect preselected wavelengths of light, thereby preventing their unwanted transmission through the filter. The operation of a conventional quarter-wave stack to block selected wavelengths is well-known to those with skill in the art. The thickness of each layer in the stack is generally one-quarter the wavelength of the light to be blocked.
Unfortunately, quarter-wave stacks suffer from several disadvantages. Because quarter-wave stacks also partially reflect wavelengths for which the layer thicknesses are odd multiples of the quarter wavelengths, very complex arrangements are required to reduce the resulting sidebands. Sidebands are the wavelengths, on either side of a particular desired reflected or blocked wavelength, which are undesirably reflected to a lesser, but significant, degree. Quarter-wave stacks also suffer from excessive thickness, especially in multiple-line rejection filters in which multiple wavelengths, or lines, are desired to be blocked. Multiple-line rejection quarter-wave stacks are generally made by series addition in which a series of quarter-wave stacks, each made to block a particular wavelength, are stacked one on top of the other.
Quarter-wave stacks are also disadvantaged by a limited availability of suitable materials in obtaining a narrow bandwidth of the desired blocked wavelength(s) and in obtaining desired material properties, such as strength and hardness, of the film.
Gradient-index films solve many of the problems with discontinuous films. They are made by a variety of methods, typically variations of evaporating film materials in a vacuum and then condensing the vapor under very controlled conditions onto the cool surface of the material to be coated or other substrate. Because the refractive index is varied almost continuously, the restrictions of discrete layers are eliminated and films can be made that provide much greater flexibility in performance characteristics. Bandwidths can be made arbitrarily narrow and sidebands greatly reduced. Additionally, materials engineering techniques can be introduced in the vacuum deposition, or other appropriate, process to provide preselected film microstructures and material properties.
The rugate filter concept, which varies, in a gradient-index film, the refractive index sinusoidally, was first developed in the mid-1970's by Conrad Phillippi and others at Wright-Patterson Air Force Base, Ohio. Rugate filters provide a number of advantages, particularly in that they make the design of seemingly complex filters much more straightforward. Sinusoidal variation results in relatively simple equations for describing the reflection characteristics of rugate filters. Generally, a desired reflected wavelength is directly proportional to the period of a simple sine wave that describes the film refractive index profile. To achieve multiple-line rejection filters, the individual sine waves for each wavelength are merely linearly superimposed in parallel addition to achieve a multi-sinusoidal refractive index profile that varies with film thickness in a manner that looks complex, but which is easily determined. Further, multiple-line rugate filters, formed by parallel addition of sine waves, are much thinner than series added quarter-wave stacks.
Unfortunately, the peak-to-peak index modulation (N.sub.p), the highest refractive index on a sine wave minus the lowest index, of a cumulative refractive index profile increases linearly with the number of lines. The total refractive index swing for a filter with N sine waves, each of which with identical N.sub.p, is N.times.N.sub.p. The total available index modulation for a pair of materials A and B is N.sub.A -N.sub.B. Therefore, the number of lines for which a given filter can be designed to block under parallel additional alone is limited by the requirement that (N.times.N.sub.p)&lt;(N.sub.A -N.sub.B). The requirements for monitoring and control of the film deposition process become more severe as the refractive index modulation becomes a large fraction of the total possible variation with given materials. The limitations of present thin film technology in monitoring and control is a primary difficulty addressed in the rugate filter prior art.
Thus it is seen that there is a need for a method for making multiple-line rugate filters that reduces peak-to-peak index modulation while substantially preserving the advantages of parallel-added multiple-line rugate filters.
It is, therefore, a principal object of the present invention to provide a method for making a multiple-line rugate filter that has the smallest possible peak-to-peak index modulation.
It is a feature of the present invention that it produces the smallest possible peak-to-peak index modulation while preserving the thinnest possible film.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.